Radio frequency identification (RFID) technology is a contactless automatic identification technology, which can be applied to warehouse management, identification, transportation, food, medicine, animal management, and other fields. Due to its wide application, RFID technology in recent years receives more and more attention, and RFID systems with high performance need to dynamically adjust amplitude limiting circuit of rectifier circuit in the radio frequency front-end to meet the performance requirements according to different applications, temperatures and RF field strengths.
RFID tags can be categorized into passive tags and active tags. The energy of a passive RFID tag is harvested from the RF carrier wave energy emitted by the reader device without an external power supply. The RF signal is received by the antenna, and passed through internal rectifying circuit and voltage regulating circuit to form a stable power source required by the reset circuit and the digital circuit of the RFID tag. The voltage regulating circuit usually requires a constant band-gap reference voltage which does not change with temperature. The typical value of the band-gap voltage is usually 1.2 volts.
However, because the voltage regulating circuit does not work at the time when the passive RFID tag is just powered up, the voltage regulating circuit can't provide power to start up the digital circuit, which then is unable to output the logic start-up control signal to start the band-gap reference circuit. Therefore the band-gap reference circuit needs to achieve the functionality of self-start.
Band-gap reference circuit has two start-up modes. The first mode is the dynamic start-up during the power-up transition process that the power ramps from low voltage to high voltage. The typical situation that this mode occurs is as follows: the antenna of the passive RFID tag system receives a radio frequency signal, the radio frequency signal is then rectified and charges the on-chip capacitor to achieve charge accumulation, thereby forming an output voltage which has a power-up transition process that ramps from low to high, and the band-gap reference circuit should be triggered to achieve self-start during the transition process.
The second mode is a static start-up after the voltage of power source is stable, which is under the normal working condition without substantial level change, and the band-gap reference circuit must depend on the start-up circuit to enter into the on-state, making itself started to output the band-gap reference voltage.
The power consumed by the start-up circuit of the band-gap reference voltage module must be as small as possible. It is even desirable to be in a dormant state that consumes zero power after the completion of start-up. However, in the band-gap reference self-starting circuit of the prior art, because the power consumption is substantially large after start-up, or because the complicated structure leads a large chip area, the requirements of the passive RFID tag can't be best satisfied.